The present invention relates to a low-noise amplifying device for use in a satellite broadcasting receiver, and particularly to an LNB (Low Noise Block downconverter), an antenna unit and the like.
FIG. 17 shows a block diagram of a typical satellite broadcasting receiving device. This satellite broadcasting receiving device is constructed of a receiving antenna 1, an outdoor unit 2, an indoor unit 3 and a display device 4 such as a television receiver or the like. The receiving antenna 1 receives and collects a faint radio wave from a broadcasting satellite and thereafter feeds the same to the outdoor unit 2. The outdoor unit 2 is constructed of an LNB (Low Noise Block Downconverter) 5, which low-noise-amplifies the faint radio wave fed from the receiving antenna 1, frequency-converts the resulting signal into an IF (Intermediate Frequency) band and supplies the resulting low-noise signal of a sufficient level to the indoor unit 3 connected as a next stage. The indoor unit 3 is constructed of a DBS (Digital Broadcasting Satellite) tuner 6, an FM Frequency Modulation) demodulator 7, a video and audio signal processing circuit 8 and an RF (Radio Frequency) modulator 9. Then, the signal is group-converted converted into a 1-GHz band by the LNB 5. A desired channel is selected from reception channels given from a coaxial cable 10 and converted into a second intermediate frequency so as to be easily processed. A baseband signal is extracted by the FM demodulator 7 and separated into a video signal and an audio signal. The separated signals are processed and RF-modulated and thereafter outputted to the display device 4.
FIG. 18 is a circuit block diagram of a LNB for domestic CS (Communication Satellite) reception that serves as a LNB for general Ku-band (10 GHz to 13 GHz) reception. An arriving signal of an input frequency of 12.2 GHz to 12.75 GHz is received by an antenna probe 12 inserted in a waveguide 11, low-noise-amplified by an LNA (Low Noise Amplifier) 13 and thereafter passed through a BPF (Band Pass Filter) 14 that allows the desired frequency band to pass, and which removes a signal in the image frequency band. The resulting signal is thereafter mixed by a mixer circuit 15 with an oscillation signal of 11.2 GHz. The oscillation signal has been outputted form a local oscillator 16, and through a BPF 17 and frequency-converted into a signal in an IF band of 1000 MHz to 1550 MHz. Then, the resulting mixed signal is amplified by an IF amplifier circuit 18 so as to have appropriate noise and gain characteristics, and is outputted from an output terminal 19. It is to be noted that the reference numeral 20 denotes a power supply.
FIG. 19 is a circuit block diagram of a LNB for domestic COMETS (Communications and Broadcasting Engineering Satellite) reception that serves as a LNB for Ka-band (17 GHz to 23 GHz) reception. An arriving signal having an input frequency of 20.4 GHz to 21.0 GHz is received by an antenna probe 22 inserted in a waveguide 21, low-noise-amplified by an LNA 23 and thereafter inputted to a mixer circuit 25 after being subjected to image removal. Then, the resulting signal is mixed by the mixer circuit 25 with an oscillation signal of 18.7 GHz; the oscillation signal having been outputted from a local oscillator 26, passed through a BPF 27 and frequency-converted into a signal of an intermediate frequency band of 1700 MHz to 2300 MHz. Then, the resulting mixed signal is amplified by an IF amplifier circuit 28 so as to ensure appropriate noise and gain characteristics, and is outputted from an output terminal 29. It is to be noted that the reference numeral 30 denotes a power supply.
In regard to the electric characteristics of the LNAs 13 and 23, the noise figure (NF) and the gain generally deteriorate and become sensitive to the characteristics of each element, to characteristics of the board pattern and to variations in structure as the frequency increases. Therefore, a stable operation inevitably becomes hard to achieve in the Ka-band as compared with the Ku-band. Accordingly, in the general process of manufacturing the LNB for Ka-band reception, as shown in FIGS. 20A and 20B, operational stability of a circuit board (PWB: Printed Wiring Board) 31 is achieved by soldering the surface of a ground pattern 32 to a base board 33, and thereafter screwing (not shown) the resulting body to a chassis 34 (i.e., upper planar surface) of the waveguide 21. In this case, a microstrip line 35 is formed on the upper surface of the PWB 31, and the upper end of an antenna pin 37 that constitutes the antenna probe 22, while being inserted in a dielectric body 36 on the chassis 34 side, is electrically connected via a metal plate 38 to the tip of the microstrip line 35.
However, the above LNB for conventional Ka-band reception has had problems as follows.
(1) In the Ka-band, a stricter method for grounding each element is required, as compared to the Ku-band. Therefore, in FIGS. 20A and 20B, the ground pattern 32 of the PWB 31 is soldered to the base board 33 and screwed to the chassis 34 when manufacturing the LNB. However, such a complicated fixation makes the manufacturing process of the LNB for Ka-band reception more difficult than that of the LNB for Ku-band reception, causing a cost increase.
(2) FIG. 21 shows characteristic impedance of a route extending from the antenna probe 22 to the LNA 23 shown in FIGS. 20A and 20B. According to FIG. 21, a portion that belongs to the antenna probe 22 and penetrates the dielectric body 36 is designed so as to have a characteristic impedance Zo of 50 .OMEGA. in a coaxial structure. However, a portion that extends through the waveguide wall to a connection to the microstrip line 35 has an inductance component because the portion cannot have a coaxial structure, with the result that a characteristic impedance Zconnect of the portion does not match 50 .OMEGA.. Therefore, the characteristic impedance Zconnect matches neither with the characteristic impedance Zo (=50 .OMEGA.) in the portion of the antenna probe 22 that touches the dielectric body 36 nor a characteristic impedance Zo (=50 .OMEGA.) on the input side of the microstrip line 35. Also, a characteristic impedance Zin of a route extending from the microstrip line 35 to the LNA 23 cannot match to 50 .OMEGA.due to existence of the LNA 23. Therefore, the characteristic impedance Zin is made to match with the Zconnect by correcting the shape of a stub 39 of the microstrip line 35. However, as described above, the LNB for Ka-band reception is sensitive to the characteristics of each element, the PWB pattern and the variation in structure. Therefore, considerable accuracy is required for the adjustment of the above stub, meaning that the matching adjustment of the LNB for Ka-band reception is very difficult.